We are investigating the methylation of the N epsilon-amino groups of the lysyl residues in the arginine-rich histones in developing rat brain. The brain is extremely rich in histone lysine methyltransferase early in life. The transfer of methyl groups to the N epsilon-amino group of lysine is a late event occurring after histones are bound to DNA. In developing rat brain a fraction of the histone molecules bound to DNA have not been fully methylated, consequently chromatin from these animals can be utilized as substrate to investigate the kinetics of methlation in vitro. The enzyme catalyzing the methylation of the lysyl residues in histones H3 and H4 is bound to chromatin; however, it can be selectively eluted by sequentially extracting chromatin with water. We have purified the enzyme some additional 50-fold through ammonia sulfate preciption, gel filtration and chromatography on DEAE-cellulose. Methodology is outlined to purify the enzyme to homogeneity. Once purified, such properties as molecular weight, substrate specificity, number of subunits, Km value for S-adenosylmethionine and inhibitor constants for the D and L isomers of S-adenosylhomocysteine will be determined. The effects of pH, polyamines and various cations on the methylation of histones bound to DNA will be determined and the validity of these results ascertained by measuring the amount of 3H-methyl incorporated into mono-, di- and trimethyllysine in histone H3 and mono-and dimethyllysine in histone H4. By preparing ferritin-labeled antibodies to the purified enzyme we may be able to determine its localization within the nucleus by electron microscopy. The effects of dietary deficiencies in methionine and folic acid on the methylation of histones will also be investigated.